In steelmaking, it is desirable to form coherent jets to promote mixing of the molten steel and to dilute the carbon monoxide (CO) in the molten steel and encourage the carbon and oxygen to come out of the steel. However, the use of oxygen to form such coherent jets can result in oxidation of the steel and undesirable by-products. Thus, it would be useful to form coherent jets from inert gases that do not react with the steel. The most desirable inert gas is argon because it is truly inert. Argon does not react at all with steel. Other inert gases are also desirable, but may have some reaction with steel. For example, nitrogen may cause “nitrogen pickup” and add nitrogen into the steel, affecting the quality of the steel. Another inert gas such as carbon dioxide may oxidize the molten steel bath due to the dissociation of CO2.
In general, the prior art teaches utilizing the “external shroud” technique, whereby the main jets, including an inert gas, are surrounded by an externally produced flame shroud. U.S. Pat. No. 7,452,401 and entitled “Oxygen Injection Method” (Mahoney et al.), discloses the “internal shroud” method to form oxygen coherent jets for application to improving the top-blown refining process of molten metal baths (e.g., basic oxygen furnace (BOF) steelmaking).
The internal shroud technique described by Mahoney et al. incorporates the following elements:                1. production of a supersonic oxygen stream in a converging-diverging nozzle;        2. blending hydrogen-containing fuel into the perimeter of the oxygen, upstream of the nozzle exit;        3. exhausting the combined supersonic stream as a jet into a furnace for example a basic oxygen furnace at high temperature; and        4. combusting the injected fuel and oxygen in the shear (or mixing) layer to produce a coherent jet.        
A problem arises when this method is applied to pure inert gas or high concentration inert gas, balance oxygen. The internal shroud method is ineffective for producing coherent jets of inert gases due to the elimination or suppression of fuel combustion in the jet shear layer (i.e., combusting the injected fuel and oxygen in the shear layer to produce a coherent jet is not possible).
Therefore, a problem to solve is the production of coherent jets containing pure or a high concentration of inert gas, particularly argon, using the internal shroud technique. Another problem to solve is the improvement of the refining of molten metal, particularly the basic oxygen process, by the application of internal shroud coherent jets containing argon.
Japanese Patent Application No. JP2002-288115 (JFE/Nippon) is concerned with the process of flame stabilization within a duct. This is accomplished by injecting fuel, which mixes with a portion of the main oxygen stream. Upon ignition, the flame is stabilized within an annular groove located in the gas passage wall, which acts as a flame holder. As a result, this technique cannot be applied to produce argon coherent jets. Japanese Patent Application No. JP2003-0324856 discloses a single burner lance capable of supplying flame and an oxygen jet to a wide area in melting/refining of iron, but does not discuss injection of an inert gas or an internal shroud.
Because of the difficulties in applying an internal shroud method to an inert gas, it has not been achieved thus far. The present invention allows the relative advantages of the internal shroud method versus the external shroud method to now be applied to inert gases such as argon.